In a living body implantable electromedical device The present invention relates to an electromedical device implantable in a living body and comprising stimulating means for the stimulation of a physiological function in the living body, stimulating mode selector means connected to said stimulating means to enable modification of said stimulation by selecting one or more out of a plurality of available&imulating modes, a power source in the form of a battery for powering said stimulating means and stimulating mode selector means, sensing means connected to said battery to enable sensing of the instantaneous battery capacity and evaluating means connected to said sensing means to establish whether said battery capacity, on a sensing event, is higher or lower than a predetermined first threshold value adapted to guarantee, in an assumed standard operation of the device, its function within a predetermined time interval during which said battery capacity shall exceed a lower second threshold value.
The invention is primarily intended for use with an electromedical device such as a pacemaker, which is intended to stimulate the cardiac function of the living body by means of generated electrical pulses different in time when said cardiac function deviates from normal.
A device of the type initially cited is disclosed by U.S. Pat. No. 390 020. This device relates to a programmable pacemaker capable of operating in several stimulating modes, having battery powered stimulating means and stimulating mode selector means. Sensing and evaluating means, which can be activated externally by means of a magnet, for example at a medical examination, monitor the terminal voltage of the battery and cause the pacemaker via the stimulating selector means to change operation from a first stimulating mode with a programmed stimulation rate to said mode with a fixed stimulation rate, when the terminal voltage decreases below a first threshold value, and to operate in a second predetermined stimulating mode at a fixed stimulation rate when the terminal voltage decreases below the first and a lower second threshold value. The limitation to a predetermined stimulating mode at a fixed rate decreases the current demand from the battery and indicates to the patient or an observing physician that the pacemaker should be replaced.
U.S. Pat. No. 4,416,282 discloses a similar pacemaker, which also includes sensing and evaluating means for monitoring of the battery capacity with regard to two battery depletion levels. The stimulation rate automatically decreases with the decreasing of the battery capacity below the depletion levels.
As is known from U.S. Pat. No. 4,606,350 or U.S. Pat. No. 4,290,429, the battery monitoring can be performed by cutting off all current flow from the battery to the pacemaker circuitry, which, however, is still powered by a by-pass capacitor, whereby a test capacitor is connected to the battery and the time period needed for the voltage across the test capacitor to reach a predetermined value gives a measure of the internal resistance of the battery.
From U.S. Pat. No. 4,481,950, another method to monitor the battery capacity is known, where the battery is loaded with a predetermined load and the battery terminal voltage is sensed. The sensed voltage gives a measure of the internal resistance of the battery, the value of which has a certain unambiguous relation to the instantantaneous battery capacity. As is known from U.S. Pat. No. 4,120,306, the internal resistance value of lithium-type batteries usually used for pacemakers is lower, the higher the battery capacity and vice versa.
When the monitoring of the battery capacity is activated as described in connection with the above mentioned U.S. Pat. No. 4,390,020, the battery capacity must be sufficiently high to guarantee operation of the pacemaker under definite conditions during a predetermined time interval up to the next medical examination; otherwise the approaching of the end of life (EOL) of the battery must be indicated, so that the physician can take necessary action in order to replace the pacemaker. As some time may pass between the sensing of low battery capacity and the point in time when the pacemaker can be replaced, a date for replacement of the pacemaker by a physician must be carefully selected. From this point in time, hereafter called elective replacement time (ERT), when the battery capacity approaches a critical first threshold value (ERT-value) till the end of life (EOL) of the battery, when the battery capacity approaches a lower second threshold value (EOL-value), the function of the pacemaker under given conditions, i.e., in an assumed standard operation, must be guaranteed. It is not unusual to have the time period between ERT and EOL, also called "safety time", set longer than three months, whereby the EOL-value can be set so low that the pacemaker ceases to function when it reaches this value or set: somewhat above this value, in which case it may be suitable to provide an alarm function.
Modern pacemakers of the type mentioned at the beginning are, however, made programmable in order to adjust stimulating mode, including other parameters such as output energy, to different physiological needs. The energy consumption of the device is hereby changed. Therefore, not only the time period from the beginning of life of the battery up to the point in time of reaching the ERT-value but also the safety time will be shorter for stimulating modes with a high energy consumption and/or at a high degree of utilization and vice versa.
Thus it appears as a technical problem that, due to a changed stimulating mode, an estimate of the EOL is no longer accurate or valid, so that during the safety time, the function of the device cannot be guaranteed. It is to be noted that even temporary drops of the battery voltage below a minimum voltage supply level are fatal; memory or parameter loss may be the result. It is an object of the present summary of the invention to achieve a constant safety time between the appearance of the ERT-value and the EOL-value.
In accordance with the invention, this object is achieved in that the sensing and evaluating means of the device specified at the beginning are arranged to vary the first threshold value (ERT-value) in dependence on the utilized stimulating mode and in dependence on the degree of utilization of previously selected stimulating modes recorded in and available from the stimulating mode selector means in such a way that a higher threshold value is selected for stimulating modes with a higher energy consumption and higher degree of utilization and a lower threshold value is selected for stimulating modes with a lower energy consumption and lower degree of utilization.
Thus, an adaptation and stabilization of the time duration between the appearing of the ERT-value and the point in time of the EOL-value is achieved according to the utilized stimulating mode, which deviates from an assumed standard stimulating mode. Thus the required safety time is always present.
According to a first embodiment of the invention, it is suggested that, on the sensing event, the battery is loaded with a standard load or a standard current, which corresponds to the load or device current consumption during the standard operation, and that the terminal voltage of the battery is compared to a reference voltage representing said first threshold value and being adjustable in dependence on the device energy consumption in the utilized stimulating mode.
The comparison can be carried out by a voltage comparator which compares the terminal voltage of the battery with said reference voltage which is generated by a digital-to-analog converter; this digital-to-analog converter is controlled by a counter which is adjustable by the stimulating selector means to a value corresponding to said first threshold value.
According to another embodiment of the invention, the battery is loaded with an adjustable load or current representing said first threshold value and corresponding to the energy consumption of the selected stimulating mode. In this case, the terminal voltage of the battery is compared with a fixed reference voltage.
The adjustable load can be made up of a number of fixed loads which are connectable to the battery, separately or together.
A further step to keep the safety time constant is, when the ERT-value appears, to limit the selection of the stimulating modes to those with a reduced energy consumption. This limitation can be adapted to the current depletion of the battery by increasing the number of stimulating modes to be inhibited for selection over a period of time. The limitation itself can also be inhibited during a predetermined delay time when a low degree of utilization of the previously selected stimulating mode is established.
An adaptation of the limitation to the current battery depletion can also be achieved by repetition of the battery test after a predetermined time duration following the appearing of the ERT-value. The battery capacity is now compared with a third threshold value between said first and second threshold values, and an additional limitation of stimulating modes is implemented when the now sensed battery capacity is below said third threshold value.